Field of the Invention
The present invention relates to an optical transmission device and a bidirectional optical transmission device.
Description of the Related Art
Conventionally, a bidirectional optical transmission device such as disclosed by Japanese Unexamined Patent Application, First Publication No. S62-3205 (hereinafter, referred to as Patent Document 1) is known.
The bidirectional optical transmission device provided with a pair of transmitting/receiving units, each of which includes an optical transmitter, an optical receiver, and a polarization combiner/splitter.
The optical transmitter emits an optical wave having a polarization state.
The optical receiver can receive an optical wave having a polarization state.
The polarization combiner/splitter can split and combine an optical wave having a different polarization state and is connected to the optical transmitter and the optical receiver.
The transmitting/receiving units which form one pair are connected to each other via a polarization-maintaining optical fiber.
In the disclosure of the above bidirectional optical transmission device, the directions of electric fields on planes of two optical waves which bidirectionally pass through the inside of one polarization-maintaining optical fiber are orthogonal to each other.
Additionally, in order to cause the directions of the electric fields on planes of the two optical waves which pass through the inside of one polarization-maintaining optical fiber as mentioned above to be orthogonal to each other, one of the optical transmitters and the other of the optical transmitters emit optical waves whose electric fields on planes are orthogonal to each other.
As a result of adopting the foregoing configuration, it is possible to reduce polarization crosstalk of the above-mentioned two optical waves.
In the configuration of the aforementioned Patent Document 1, a polarization state of the optical wave emitted from one of the optical transmitters is different from a polarization state of the optical wave emitted from the other of the optical transmitters.
Because of this, for a planar optical waveguide device that includes one of the optical transmitters and a planar optical waveguide device that includes the other of the optical transmitters, it is necessary to prepare different kinds of planar optical waveguide devices.
In the above-described manner, in the case of using a pair of two planar optical waveguide devices which are different from each other, the cost of manufacturing a bidirectional optical transmission device increases or ease of maintenance is deteriorated.
Furthermore, in this kind of bidirectional optical transmission device, there is a demand that it is possible to achieve larger capacity optical transmission by using a planar optical waveguide device including a substrate on which a plurality of transmitting/receiving units are arranged.
In this case, a pair of planar optical waveguide devices, each of which includes a substrate on which a plurality of transmitting/receiving units are arranged in parallel, may be used, and the transmitting/receiving units corresponding thereto may be connected each other via a polarization-maintaining optical fiber.
In the case where a plurality of transmitting/receiving units are provided, it is conceivable that transmission is carried out in the same polarization-maintaining optical fiber by using two modes.
In contrast, in the case where two modes are used for transmission and reception, it is possible to reduce an effect of polarization crosstalk between two modes.
However, in the case of carrying out transmission and reception inside one polarization-maintaining optical fiber described above, for example, in the case of adopting a configuration in which an optical wave having horizontal electric field (transverse-electric, TE) mode is output from each optical transmitter of one of the planar optical waveguide devices, each optical receiver of the other of the planar optical waveguide devices which corresponds thereto is configured to able to receive an optical wave having TE mode.
For this reason, in order to reduce polarization crosstalk, the configuration is necessary so that an optical wave having a transverse magnetic field (transverse-magnetic, TM) mode orthogonal to TE mode in the electric field on a plane is output from each optical transmitter of the other of the planar optical waveguide devices.
Accordingly, also in this case, it is necessary to prepare two planar optical waveguide devices which are different from each other, not only the manufacturing cost increases but also flexibility of maintenance or modification of wiring is limited.